<!DOCTYPE html>

<html lang="en">
    <head>
        <meta charset="UTF-8">
        <title>优先级</title>
        <style type="text/css">

            #test { border: 1px solid green ; }

            .first {
                width: 500px ; height: 100px ; border : 1px solid blue ;
            }

            div {
                border: 1px solid red ;
            }
        </style>
    </head>
    <body>

        <div class="first" id="test" style=" border: 1px solid gray ; "></div>

    </body>
</html>